1. Field of the Invention
The present invention relates to a device for connection between a rail for fuel under pressure and at least one injector, for an internal-combustion engine.
2. Description of the Related Art
As is known, in injection engines the rail for fuel under pressure, common for all the injectors, is connected to the injectors themselves by means of metal tubes. The rail has a tubular shape and has an external diameter in the region of 30 mm, an internal diameter in the region of 10 mm, and a weight of approximately 3 kg. The tubes are normally connected to the rail by welding and have the only function of hydraulic connection. In turn, the rail is fixed on the engine block by means of an appropriate supporting plate. Both this plate and the rail require a considerable space in the engine compartment, so that their placing on the engine is rather complicated, and the injection system proves relatively heavy and costly.
From the document No. EP 0866 221 A1, a common rail for fuel is known having a reduced diameter, on which the tubes have an end with oversized external diameter, i.e., a swelling, which engages in a fluid-tight way a conical seat present on the rail. Each tube is fixed via a connection element carried by the rail, screwed on which is a ring nut designed to press directly on the swelling of the tube, to ensure tightness thereof with the conical seat. The connection element is fixed on the rail by welding, or force fitted.
The above connection device presents various drawbacks. In the first place, since the diameter of the rail is rather contained to reduce its weight, cost and overall dimensions, and since the size of the swelling of the tube is not smaller than a minimum diameter of encumbrance, the conical seat designed to house the swelling of the tube entails a diameter to ensure tightness of the swelling on the rail at a radial level of the rail that is too external. On account of the high operating pressures, said fit consequently proves structurally critical.
In addition, since the swelling of the tube is in direct contact with the rail, without any intermediate connection, it is not possible to set, between the rail itself and each tube, a calibrated restriction for hydraulically uncoupling the tube from the rail. The direct seal of the swelling of the tube on the rail penalizes the flexibility of the system, so that any even slight misalignment of the axis of the tube with respect to the axis of the conical seat of the rail, for example due to normal activities of maintenance of the engine, could jeopardize its tightness.
Finally, since in couplings for high-pressure tightness between two elements it is often necessary to adopt materials with different hardnesses to improve tightness thereof, in the initial assembly the element made of softer material undergoes permanent plastic deformations. Consequently, should the two elements be uncoupled, it would no longer be possible to guarantee tightness in the subsequent assembly. In general, the softer material is adopted for the less costly component, which must then be replaced whenever it is uncoupled. It is evident that, if for reasons of simple maintenance the tubes were to be removed from the rail, it would be necessary to replace also the tubes or the rail, according to the choice made for the element of softer material, with evident prejudice from the economic standpoint.